The invention relates to a method and an apparatus for recombining a first sequence of a digital data words occuring in a first ordering together with correction words into a second sequence of digital data words occuring in a second ordering, in which a given number of data words and correction words compose blocks.
Such methods and implementations are used in particular in the reproduction of digitally recorded audio signals on a recording medium. It is common practice to digitize originally analog signals for their recording on a recording medium. This means that the signal then consists of a sequence of data words, each of which representing a sample, for example the instantaneous value of the voltage of an originally analog signal. In order to protect the recorded data words, i.e. in order to prevent that these data words are modified and in order to prevent that several consecutive data words are destroyed at the same time, the sequence of data words is coded prior to the recording according to well-known methods. In the coding process, the original sequence of the data words is modified according to the rules of a code, and additional correction words are added. The sequence of the data words in this operation is modified in such a way that data words which were originally consecutive are separated by some distance and that other data words are placed between them. The correction words make it possible to correct and thus preserve one or several of the data words in a group. The coding of the data words makes it possible to protect the recordings from disturbances caused by dust, damage of the recording medium, cuts in the case of a tape recording, editing operations etc. In the process of reproducing the recorded data words, these must be decoded according to the rules of the same code. In the decoding process, the original sequence of the data words is reconstructed. Efficient use of the correction words which have been added during the recording process makes it possible to reduce the effects of the above-mentioned perturbations, so that the original digital signal can be reproduced with as few errors as possible.
Such decoding methods are known from the Audio Engineering Society Preprints of Oct. 31st, 1980, Nr. 1677, and Mar. 2nd, 1982, Nr. 1885. The individual data words which represent, for example, samples of an audio signal, are interleaved; parity check sums are added, and the data words are formatted into blocks. In the case of the well-known DASH format, this means that 4 parity check sums, 2 of them Q and 2 of them P, as well as a word for error detection, are added to 12 data words.
In decoding according to state-of-the-art methods, a number of processing steps are common.
1. processing of the block error detection PA1 2. processing of the 1st correction code PA1 3. processing of the 2nd correction code
In this sequence, each previous code is used as an error indicator for the following one.
The repeated evaluation of the 1st and the 2nd correction code makes it possible to improve the correction capability of the method and its implementation. However, the effect of multiple correction steps remains limited. Without an error detection performed by the code for block error detection, it is only possible to correct 2 consecutive blocks with 2 processing steps.